Modulation booster



Mas'ch 16, 1948. F. H. sLAYMAKl-:R 2,437,839

MODULATION BOOSTER Filed Aprn s, 194e s sheets-sheet 2 AM MA PL Hr: cueeE/vr INVESTOR. FPHNK h'. SL HYMHKE A QRNEYS @Ich 16, 1948. F. H. sLAYMAKl-:R

MODULATION BOOSTER Filed April s, 194e 3 Sheets-Sheet 3 OUTPUT INPUT N01 INPUT No.2

ATTORNEYS Patented Mar. 16, 1948 MODULATION BOOSTER Frank H. Slaymaker, Rochester, N. Y., assignor to Stromberg-Carlson Company, Rochester, N. Y., a corporation of New York Application April 3, 1946, Serial N o. 659,229

Claims.

The present invention relates generally to signal modulated high frequency transmission systems and more particularly to a modulation booster for boosting the degree of modulation.

This application is a continuation in part of the invention disclosed and claimed in Patent 2,403,245, issued July 2, 1946, and assigned to the same assignee as the present invention.

While my invention is applicable to and usable in any system where it is desirable to increase the modulation intensity, it nds particular utility in the intra-tank communication system disclosed in Patent 2,362,692, of Harold Goldberg, J. Russell Stewart and myself, issued November 14, 1944, and assigned to the same assignee as the present invention. I

It is, therefore, one object of my invention to handle carrier currents which are very weakly modulated Without overloading the amplifying equipment.

Another object of my invention is the increase in modulation intensity from a very low degree, say 3 per cent, to a very high degree, say 10S per cent. i

Still another object of my invention is to increase the modulation of the radio frequency carrier by operating on the carrier envelope itself without requiring the use of audio frequency in securing the increase.

Various other objects and advantages of the invention will appear from the following description when read in conjunction with the appended drawings, in which:

Figure 1 is a block diagram illustrating the above mentioned intratank communicating system, with the modulation booster connected therein;

Figure 2 illustrates, in diagrammatic form, a

typical carrier frequency transmission system, having my modulation booster incorporated therein; Figure 3 shows the fundamental, schematic circuit of the modulation booster, with the bypass filters, filament supply and other parts of the circuit, not necessary for illustrating the present invention, omitted;

Figures 4 and 7 show the control grid voltageplate current characteristic for a tube used as a modulation booster in accordance with my invention.

Figure 7A shows the potential acrossthe tuned plate load of the circuit of Figure 3 and it will be noted that these waves are modulated to a greater degree than those of the input carrier of Figure 7. It will be understood that the output load may be tuned to any desired multiple of the input frequency to provide any desired harmonic of the input frequency in the output circuit, as later explained.

Figures 5, 6 and 8 are curves which will be referredto in pointing out various features and advantages of the present invention; and,

Figure 9 shows a modication of the circuit shown in Figure 3.

Referring now to Figure 1, LI and L3 represent two antennas strung around the inside of a tank or other mobile unit. L2 and L4 indicate two portable loop antennas which are carried in the clothing, such as being sewed in the users belt, of the members of the tank crew. The portable modulator comprises a telephone transmitter (not shown) which functions to modulate a tuned circuit comprising a condenser (not shown) and loop L2. The portable receiver comprises a telephone headset and a detector (not shown) connected to a tuned circuit comprising a condenser (not shown) and loop L4.

Oscillator OSC may be an electronic oscillator which supplies radio frequency oscillations FI to radio frequency amplifier R. F. AMP, the output of which is connected to loop LI. A radio frequency electromagnetic iield is therefore set up in the space within loop Ll, which field is absorptively modulated in accordance with the variations provided by the portable modulator, all as disclosed in the above mentioned Patent 2,362,- 692.

In any system where the modulated radio frequencies are weakly modulated, as for example in the system illustrated, these radio frequencies may be connected to the circuit 2l), illustrated in Figure 3 and the per cent modulation boosted. The potential of battery C is such that grid G is biased to an operating point such as 2| of Figure 4 or Figure 7, whereby one edge of the modulated radio frequency or carrier envelope will fall on the linear portion of the Eg-Ip curve. The plate current in the output of tube V will then consist of a series of unidirectional pulses varying in amplitude with the modulation and occurring at the same frequency as the carrier. Furthermore, the various harmonics of the carrier fundamental frequency are also present in the plate current.

The plate load, comprising inductance L and capacity C, may be tuned to the input frequency FI, or any of its harmonics, and the potential across the output terminals of the tube V will consist of a true modulated radio frequency wave, but modulated at a much higher degree or percentage than the applied radio frequency in the input. This boost is illustrated by the envelopes ofthe input and output waves in Figures '7 and 7A. These boosted waves appearing across the output terminals of Figure 3 are applied to the input of the radio frequency amplifier of Figure 1 which has its output connected to loop L3. By tuning the LC circuit of Figure 3 to the second harmonic of the oscillator frequency Fl, a frequency'F2, which is double that of FI, is applied to the R. F. amplifier whose output is connected to antenna L3. Thus, the boosted modulations are carried by carrier F2 which is twice the frequency of FI and are then received and detected Aby loopL4 and the portable receiver, the loop clrcuit'being tuned to frequency F2.

From the above explanation and with particular reference to Figure 7, it will be noted that a series of pulses, of the same frequency as vthe input carrier, appear in the plate current of tube V in Figure 3. Furthermore, since-.the 4pulsesLin the plate circuit are not sinusoidal, harmonics: are present and by tuning the outputcircuit tothe fundamental or any desired harmonic thereof, it is possible to extend the modulatedfundamental, or harmonic, to a Work circuitgsuchfas .the;R.;F. amplifier 22 of Figure 1. It` is an observedfact that the harmonics in the output'circuit are"modulated to substantially the same degree orper cent boost as the fundamental.

It will be seen from'Figure 4 that, While the modulation has been considerably increased, -it is not nearly 100 per cent. Modulation of the order of 100 per cent may ,be obtained-by adjusting the'grid bias C so that the Adips in the modu- 2 lated portion of the carrier-.FI Vfall exactly at the cut-off point on the Eg-Ip vcurve as illustrated in Figure 7. rFhis may also Abe Vobtained by'adiusting the amplitude of the input'potentialsfso that the dips in the modulated portion lofthe :carrier FI fall exactly at the `cut-'01T point onztheEg-Ip curve as illustrated in Figure .8. Ofcourse'fserious distortion will be evidenced if thexabovementioned dips fall back of the. cut-off point :of-'the characteristic curve.

As long as the average radio 'frequenc-yinput to the modulation booster remains constant,:iixe`d bias for the booster stage can be used. Likewise, other types of bias can be used. If the `average carrier amplitude is likely to changefspontaneously, iixed bias may not be desirable since overmodulation of the output can occur if the carrier strength drops enough to place the modulation dips back of cut-olf. The use of'a selfbiasing scheme, such as a grid leak bias, withthe grid leak and condenser having a sufficiently long time constant to prevent the bias from following :the modulation, will eliminate many of the difficulties arising from a shifting input intensity.

With a grid leak bias, the peaks in the modulation come just on the edge of zero biasfregardless of the input intensity. 'When the output is to be modulated 100 per cent, either thev absolute amplitude of the input modulation should -lie such that the dips fall at cut-offorithe-position of the cut-off pointshould be moved until it -coincides with the 'dipsin the modulation. "The position of the cut-oif point can be controlled by using a pentode for the tube V of 'Figure =3: and

adjusting the screen or suppressor :grid poten# tials.

As an example of the above described control, Figures 6 and 8 are illustrative. VFiguref4 shows the cut-off set at a value such that it is fata remote point with respect to the modulation dips,

vplished by an increase in the per cent modulation. .Inthis case the modulation is boosted to around per cent.

-It'hasf already been explained that, in addition -to the ffundamental radio frequency carrier in thefoutput the plate circuit also contains harmonies .of the fundamental. The output circuit may, therefore, be tuned to any of these harmonics'and the tube then serves as both a modulation booster and a frequency multiplier', For example, the fundametal frequency Fl (say, ,1100 kyo.) supplied by oscillator OSC in Figure lmay be'modulated very weakly in the above mentioned intra-tank communicaticn'system, this frequency is then multiplied to 2200 k. c. and the modulation boosted by the .modulation booster, .so that the signal maybe-.satisfactorily receivedby the receiver tuned to 2200 k. c.

s As long as the modulated portion of the radio frequency envelope falls on the linear portion of the Eg-Ip characteristic curve there vis no troublesome distortion introduced into the modulation of the various'harmonics and the modulation percentage'is substantially the same at the harmonicsias .at the fundamental.

I am aware that frequency multipliers-are old in the art, butas'farasl know they have been used only on unmodulated carriers because it was 4feltthat such multipliers would'introduce serious modulation distortion if an attempt was made to multiply ia :modulated carrier. I am cognizantoffthe factthat, in my sytsem, .distortion will appear if themodulatedportion of the carrier does not fall on the linear portion of the characteristic vcurve of thetube, but with an input of "a very low Ypercentage of t modulation it can veryreadilybe made tofall onlthis portion of the curve.

Another application of my modulation booster system is that it may serve-asa conventional grid modulated class C amplifier, -in Vaddition to its function as a modulation booster and frequency multiplier. To accomplish this, an audio frequency can be addedV to the radio frequency input of Figure 3 rand applied to the grid circuit, as shown by the lower curve `of Figure 5. This audio frequency will, in effect, swing the operating point of the tube and the output circuit will contain a radio frequency, modulated with the audio frequency signal, as illustrated in Figure 5.

In order to simplify the drawings, only one edge'of the input carrier is illustrated in 'Figures l6 and 8. Since the omitted portion of the carrier falls-back `of cut-off, it is believed unnecessary toshow it.

The circuit of Figure 9 is similar to that shown in Figure v3. However, a potentiometer? is provided-tozpermitvariation*of the bias on tube or discharge 'device 'V and there are provided two sets of input connections as indicated, so that one or two microphones, for-example, can be usedl at any onetime. 4The resulting modulation envelope consists of the modulation from one or both sources of signal input.

It will be understood that cathodemodulation This explanation of my invention has dealt l primarily with operation in connection with an intra-tank communication system, in which av very low degree of modulation is primarily produced and then stepped up or boosted to a very high degree, without the introduction of distortion, and operating on the high radio frequency currents. Application of this invention may, however, be found in other classes of service, such as in the radio art or in carrier system telephony. For example, in radio broadcast transmitters, particular care is required to design a class C stage which can be modulated to 100 per cent without introducing distortion, but by removing the necessity of securing 100 per cent in the modulated stage, distortionless modulation is secured and a weakly modulated carrier can be fed into my modulation booster to secure 100 per cent modulation. Such an expedient would require a comparatively small amount of audio power and the desired radio frequency power could be secured by either class A or class B amplifiers following the modulation booster.

While I have gone into considerable detail in pointing out the various features of my invention, it will be apparent that many modifications thereof might suggest themselves to those skilled in the art and I accordingly do not desire to limit my invention to the specic details disclosed except as may be necessitated by the prior art and the appended claims.

What I claim is:

1. Means for producing a high degree of modulation of a source of carrier Waves, a plurality of amplifiers of the space current type connected in tandem, means associated with said ampliers and normally effective to prevent the flow of space current therethrough, an output circuit for the first amplifier in tandem tuned to the frequency of said source, means for coupling said source to said first amplifier, an output circuit for the last amplifier in tandem tuned to a harmonic of the frequency of said source, and means for appreciably increasing the percentage of modulation between said source of carrier waves and said harmonic of said source.

2. In a high frequency transmitting system including a thermionic valve, a source of unmodulated high frequency waves, means for modulating said high frequency waves to a comparatively low degree, a biasing sourcefor supplying a fixed negative potential to the control electrode of said valve, means for connecting said sources to the control electrode of said Valve, and means controlled by the relationship between said sources for providing high frequency waves in a circuit connected to the output of said valve which are modulated to a comparatively high degree and which are of a frequency which isa harmonic of said high frequency.

3. In a thermionic amplifier, a cathode, an anode, a control electrode, input and output circuits and a source of current in said output circuit, 'means for applying a series of modulated high requency impulses to said control electrode, means for applying a negative potential to said control electrode which is so related to said high frequency impulses that only the positive and modulated portion of said high frequency impulses are effective to liberate positive impulses in said output circuit whereby said positive iinpulses are modulated toa greater degree than said high frequency impulses, means for swinging the negative potential applied to said control electrode for providing the modulation of said high frequency impulses, and means for tuning said output circuit for providing positive impulses which are a multiple of the frequency of said high frequency impulses.

4. In a thermionic amplier, a cathode, an anode, a control electrode, input and output circuits and a source of current in said output circuit, means for applying a plurality of series of modulated high frequency impulses to said control electrode, means for applying a negative potential to said control electrode which is so related to said high frequency impulses that only the positive and modulated portion of said high frequency impulses are effective to liberate positive impulses in said output circuit whereby said positive impulses are modulated to a greater degree than said high frequency impulses, means for swinging the negative potential applied to said control electrode for providing the modulation of said high frequency impulses, and means for tuning said output circuit for providing positive impulses which are a multiple of the frequency of said high frequency impulses.

5. In a thermionic amplifier, a cathode, an anode, a control electrode, input and output circuits and a source of current in said output circuit, means for applying a series of modulated high frequency impulses to said control electrode, means for applying an adjustable negative potential to said control electrode which is so related to said high frequency impulses that only the positive and modulated portion of said high frequency impulses are eiiective to liberate positive impulses in said output circuit whereby said positive impulses are modulated to a greater degree than said high frequency impulses, means for swinging the negative potential applied to said control electrode for providing the modulation of said high frequency impulses, and means for tuning said output circuit for providing positive iinpulses which are a multiple of the frequency of said high frequency impulses.

FRANK H. SLAYMAKERJ.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 1,687,245 Heising Oct. 9, 1928 2,215,672 Terman Sept. 24, 1940 2,403,245 Slaymaker July 2, 1946 

